Osteofibrous dysplasia is a rare benign fibro-osseous lesion affecting the anterior tibial cortex in 85-90% of cases, with peak incidence in the first decade of life (mean age 4-8 years). Radiographs show an intracortical lucent lesion with sclerotic borders causing anterior tibial bowing. The critical differential is adamantinoma - osteofibrous dysplasia is cytokeratin negative while adamantinoma is positive. Most cases are managed conservatively with observation as spontaneous resolution commonly occurs after skeletal maturity. Surgery is reserved for pathological fracture (10-15%) or progressive deformity.

Classic Presentation:

• 6-year-old child with painless anterior tibial swelling
• Incidental radiographic finding during trauma workup
• Anterior tibial cortex involvement with intracortical lucency
• Well-defined sclerotic margins on plain films

Diagnosis:

• X-ray: Intracortical lucent lesion with sclerotic rim
• CT: Cortical thinning and expansion without soft tissue mass
• MRI: Low T1, variable T2 signal, no soft tissue component
• Biopsy: Fibrous tissue with osteoid rimming, cytokeratin negative

Management Algorithm:

1. Initial Assessment: Clinical examination, plain radiographs
2. Advanced Imaging: CT/MRI to rule out adamantinoma
3. Biopsy Consideration: If atypical features or age greater than 10 years
4. Observation: Serial radiographs every 6-12 months
5. Surgical Intervention: Only for pathological fracture or progressive deformity

Demographics

Age Distribution:

• Peak incidence: 4-8 years (mean age 6 years)
• 90% diagnosed before age 10 years
• Rare in adults (usually progression from childhood)
• No gender predilection (equal male to female ratio)

Anatomical Location:

• Tibia: 85-90% of cases (anterior cortex diaphysis/metaphysis)
• Fibula: 8-10% of cases
• Radius/ulna: Less than 2% (extremely rare)
• Other bones: Case reports only

Incidence:

• Extremely rare: Less than 200 cases in literature
• Represents less than 0.2% of benign bone tumors
• Geographic distribution: No known predilection

Osteofibrous dysplasia is a rare benign fibro-osseous lesion affecting the anterior tibial cortex in 85-90% of cases, with peak incidence in the first decade of life (mean age 4-8 years). Radiographs show an intracortical lucent lesion with sclerotic borders causing anterior tibial bowing. The critical differential is adamantinoma - osteofibrous dysplasia is cytokeratin negative while adamantinoma is positive. Most cases are managed conservatively with observation as spontaneous resolution commonly occurs after skeletal maturity. Surgery is reserved for pathological fracture (10-15%) or progressive deformity. Biopsy should be considered if atypical features are present or age is greater than 10 years.

Pathogenesis

Proposed Mechanisms:

1. Developmental Theory: Defect in cortical bone maturation
2. Reactive Process: Response to microtrauma in growing bone
3. Mesenchymal Origin: Abnormal differentiation of osteoblastic cells
4. Genetic Factors: No consistent chromosomal abnormalities identified

Histological Evolution:

• Active phase (childhood): High cellularity, prominent osteoid production
• Quiescent phase (adolescence): Reduced cellularity, increased bone formation
• Resolution phase (post-skeletal maturity): Fibrous tissue replacement with mature bone

Relationship to Adamantinoma:

• Historical debate: Same disease spectrum versus distinct entities
• Current consensus: Separate diagnoses with different prognosis
• Key distinction: Cytokeratin expression (negative in OFD, positive in adamantinoma)
• Rare transformation: Less than 5% risk of adamantinoma-like changes

Symptoms & Signs

Typical Presentation:

• Asymptomatic (40-50%): Incidental radiographic finding
• Painless Swelling (30-40%): Anterior tibial prominence
• Activity-Related Discomfort (20-30%): Mild aching with exercise
• Pathological Fracture (10-15%): Acute pain and deformity

Physical Examination:

• Anterior tibial cortex: Firm, non-tender mass
• Overlying skin: Normal, no warmth or erythema
• Anterior tibial bow: May be present with long-standing lesion
• Neurovascular status: Always normal
• Range of motion: Full and painless at adjacent joints

Deformity Patterns:

• Anterior tibial bowing (15-20% cases)
• Progressive with growth in active lesions
• Typically mild (less than 15 degrees angulation)
• Rarely requires corrective osteotomy

Clinical Course

Natural History:

• Childhood (Active Phase): Slow expansion with growth
• Adolescence: Stabilization in most cases
• Skeletal Maturity: Spontaneous resolution common
• Adult Follow-up: Residual cortical thickening, no active lesion

Complications:

• Pathological fracture: 10-15% incidence
• Progressive deformity: 5-10% requiring intervention
• Recurrence after curettage: 50-60% (high rate)
• Malignant transformation: Extremely rare (less than 1%)

Plain Radiography

Characteristic Features:

• Location: Anterior tibial cortex, eccentric intracortical lesion
• Appearance: Well-defined lucent area with sclerotic margins
• Pattern: Geographic bone destruction (Lodwick grade IA)
• Cortical Changes: Thinning, expansion, scalloping of inner cortex
• Periosteal Reaction: Minimal to absent in uncomplicated cases

Differential Radiographic Features:

Advanced Imaging

Computed Tomography (CT):

• Cortical Assessment: Precise delineation of cortical involvement
• Matrix: No calcification or ossification pattern
• 3D Reconstruction: Useful for surgical planning if needed
• Soft Tissue: Confirms absence of extraosseous component

Magnetic Resonance Imaging (MRI):

• T1-Weighted: Low to intermediate signal intensity
• T2-Weighted: Variable signal (depends on fibrous versus osseous ratio)
• STIR Sequences: High signal in active lesions
• Contrast Enhancement: Moderate enhancement in active lesions
• Purpose: Exclude soft tissue mass (adamantinoma concern)

Nuclear Medicine:

• Bone Scan: Increased uptake in active lesions
• Clinical Utility: Limited, not routinely required
• PET Scan: Not indicated for benign lesion

Key Point for Exams: The main imaging goal is to differentiate osteofibrous dysplasia from adamantinoma. MRI showing NO soft tissue component strongly supports OFD diagnosis. If there is ANY soft tissue mass, you must suspect adamantinoma and proceed to biopsy.

Histopathology

Macroscopic Findings:

• Well-circumscribed fibrous tissue within cortex
• Gritty texture due to osteoid formation
• No cystic changes or hemorrhage
• Sharp demarcation from surrounding bone

Microscopic Features:

• Fibrous Stroma: Spindle cell proliferation, moderate cellularity
• Bone Formation: Trabeculae rimmed by osteoblasts (zoning phenomenon)
• Osteoid Deposition: Irregular seams surrounding fibrous tissue
• Cellular Atypia: None (bland spindle cells)
• Mitotic Activity: Rare to absent

Immunohistochemistry:

• Cytokeratin (AE1/AE3): NEGATIVE (key distinguishing feature)
• Epithelial Membrane Antigen (EMA): NEGATIVE
• S100: Variable positivity in fibrous component
• CD34: May be positive in stromal cells
• Ki-67 Index: Low (less than 5%)

Differential Diagnosis:

Conservative Treatment

Observation Protocol:

• Indications: Asymptomatic, stable radiographic appearance, age less than 15 years
• Follow-up Schedule:
  • First year: Every 3-6 months with X-rays
  • Years 2-5: Every 6-12 months
  • Until skeletal maturity: Annual radiographs
  • Post-maturity: Discharge if stable

Activity Modification:

• No specific restrictions for asymptomatic lesions
• Avoid high-impact sports if cortical thinning severe (greater than 50%)
• Return to activities after pathological fracture healing

Prognosis with Observation:

• Spontaneous resolution: 40-60% after skeletal maturity
• Stabilization: 30-40% with no further progression
• Progression: 10-20% may require intervention

Surgical Management

Indications:

1. Pathological fracture (not healing with conservative treatment)
2. Progressive deformity (greater than 20 degrees angulation)
3. Diagnostic uncertainty (cannot exclude adamantinoma)
4. Persistent pain affecting quality of life
5. Patient/family preference after counseling

Surgical Options:

Curettage and Bone Grafting:

• Technique: Cortical window, thorough curettage, autograft or allograft
• Recurrence Rate: 50-60% (very high)
• Indications: Small lesions, pathological fracture
• Outcomes: Often requires repeat procedures

En Bloc Resection:

• Technique: Wide resection with intercalary reconstruction
• Indications: Concern for adamantinoma, failed multiple curettages
• Recurrence: Less than 5% (definitive treatment)
• Morbidity: Significant, reconstruction challenges

Corrective Osteotomy:

• Indications: Established deformity (greater than 20 degrees) after lesion inactive
• Technique: Closing wedge or dome osteotomy
• Timing: After skeletal maturity preferred
• Outcomes: Good correction, does not address underlying lesion

Treatment Algorithm

Decision-Making Framework:

1. Initial Diagnosis (Age less than 10, typical radiographs):

   • Observation with serial imaging
   • No biopsy if classic presentation

2. Atypical Features (Age greater than 10, soft tissue mass, aggressive appearance):

   • MRI to assess soft tissue component
   • Biopsy mandatory (exclude adamantinoma)
   • Consider en bloc resection if adamantinoma-like features

3. Pathological Fracture:

   • Conservative fracture management first
   • Consider curettage if non-union after 6 months
   • Bone grafting to augment healing

4. Progressive Deformity:

   • Observation until skeletal maturity if mild (less than 15 degrees)
   • Corrective osteotomy after maturity if significant (greater than 20 degrees)
   • Address lesion separately if still active

Complications

Pathological Fracture (10-15%):

• Usually after minor trauma
• Healing: 90% with conservative management (cast immobilization)
• Non-union: Rare (less than 5%), may require surgery
• Refracture: Possible if lesion remains active

Deformity (15-20%):

• Anterior tibial bowing most common
• Progressive with skeletal growth
• Usually mild (less than 15 degrees)
• Functional impact: Minimal in most cases

Recurrence After Surgery (50-60%):

• Highest in children under 10 years
• Multiple recurrences common
• May require en bloc resection ultimately

Malignant Transformation (Less than 1%):

• Extremely rare, case reports only
• Usually represents misdiagnosis of adamantinoma
• Requires careful histological review

Prognosis

Excellent Overall:

• Benign disease with spontaneous resolution potential
• No metastatic potential
• Normal life expectancy
• Minimal functional impairment in vast majority

Long-term Outcomes:

• 70-80% stable or resolved by age 20 years
• 10-20% residual asymptomatic cortical thickening
• 5-10% require surgical intervention for complications
• Less than 5% develop adamantinoma (likely pre-existing)

Key Differentials

Adamantinoma:

• Age: 20-40 years (vs less than 10 for OFD)
• Location: Any tibial cortex, may be multicentric
• Radiology: Less well-defined, may have soft tissue mass
• Histology: Cytokeratin POSITIVE (epithelial component)
• Prognosis: Malignant potential, requires wide resection

Fibrous Dysplasia:

• Location: Medullary, not intracortical
• Radiology: Ground-glass matrix, expansile
• Age: Adolescents/young adults typically
• Histology: "Chinese characters" pattern, no osteoblast rimming
• Behavior: Does not resolve spontaneously

Ossifying Fibroma:

• Location: Mandible/maxilla primarily, rare in long bones
• Radiology: Well-defined radiolucent to radiopaque
• Histology: Spherical ossicles (psammomatoid bodies)
• Age: Young adults

Stress Fracture:

• Clinical: Activity-related pain, acute onset
• Radiology: Linear lucency, periosteal reaction
• MRI: Marrow edema, cortical fracture line
• Evolution: Healing within 6-8 weeks

Non-Ossifying Fibroma:

• Location: Metaphysis, eccentric, medullary
• Radiology: Sclerotic scalloped margins, cortical thinning
• Age: Adolescents (10-15 years)
• Histology: Storiform pattern, foam cells, hemosiderin

Australian Epidemiology and Practice

Australian Bone Tumour Registry:

• Osteofibrous dysplasia is extremely rare in Australia with fewer than 5 new cases reported annually
• Cases are typically managed through tertiary paediatric orthopaedic tumour services
• Australian data mirrors international literature with predominant anterior tibial involvement and paediatric presentation

RACS Orthopaedic Training Relevance:

• Osteofibrous dysplasia is a core FRACS Orthopaedic Oncology examination topic
• Viva scenarios commonly test differentiation from adamantinoma and fibrous dysplasia
• Key exam focus: cytokeratin immunohistochemistry, natural history, and conservative management rationale
• Examiners expect knowledge of high recurrence rate after curettage and observation-first approach

Australian Tertiary Referral Pathway:

• All suspected bone tumours should be referred to an accredited bone tumour service
• Australian Sarcoma Group (ASG) provides guidelines for benign and malignant bone tumour management
• Multidisciplinary team review (orthopaedic oncology, radiology, pathology) essential before intervention
• Paediatric cases managed at children's hospitals with dedicated tumour services (Royal Children's Hospital Melbourne, Westmead Children's Sydney, Queensland Children's Brisbane)

eTG Relevance:

• No specific Therapeutic Guidelines for this rare benign lesion
• Pathological fracture management follows standard paediatric fracture protocols
• Prophylactic antibiotic guidelines apply for any surgical intervention

Training and Education:

• Bone tumour rotation typically at major orthopaedic oncology centres during FRACS training
• Understanding differential diagnosis of fibrous lesions is essential for Part 1 Basic Sciences and Part 2 Clinical examinations
• Case presentations may appear in ISAWE format testing diagnostic reasoning